Preeclampsia is a syndrome of hypertension, edema, and proteinuria that affects 5 to 10% of pregnancies and results in substantial maternal and fetal morbidity and mortality. Preeclampsia accounts for 200,000 maternal deaths worldwide per year. Clinical symptoms of preeclampsia typically appear after the 20th week of pregnancy and are usually detected during routine evaluation of a woman's blood pressure and testing for the presence of protein in a sample of her urine. However, such clinical evaluation is ineffective for early diagnosis of the syndrome. Being able to evaluate the likelihood of developing preeclampasia, and/or being able to diagnose preeclampsia in an early stage of gestation, and/or being able to diagnose preeclampsia during a phase of the disease when clinical evaluation is uninformative, would allow early intervention and reduce the risk of medical complications and mortality for a pregnant woman and developing fetus.
Currently there are no known cures for preeclampsia. Preeclampsia can vary in severity from mild to life threatening. Maternal complications include renal failure, HELLP syndrome (hemolysis, elevated liver enzymes, and thrombocytopenia), liver failure, cerebral edema with seizures and rarely death. Potential fetal complications include low birth weight, prematurity and death. A mild form of preeclampsia can be treated with bed rest and frequent monitoring. For moderate to severe cases, hospitalization is recommended and blood pressure medication or anticonvulsant medications to prevent seizures are prescribed. If the condition becomes life threatening to the mother or the baby, the pregnancy is terminated and the baby is delivered pre-term.
Molecular mechanisms associated with preeclampsia have recently been reviewed (Mutter and Karumanchi, Microvascular Research 75:1-8, 2008). As stated by Mutter and Karumanchi, it is believed that endothelial dysfunction contributes to the clinical syndrome of preeclampsia (Roberts and Cooper, Lancet 357:53-56, 2001). Many of the symptoms of the disease may result from aberrant endothelial function (including arterial hyperreactivity to exogenous and endogenous stimuli, proteinuria related to increased glomerular permeability, cerebral edema and increased CNS permeability, as well as vascular thrombosis resulting in the HELLP syndrome) (Roberts, Semin Reprod Endocrinol 16:5-15, 1998; Roberts and Cooper, Lancet 357:53-56, 2001). As such, there has been an active search for circulating factors that cause or contribute to endothelial dysfunction.
Proper development of the fetus and placenta is mediated by several growth factors. One of these growth factors is vascular endothelial growth factor (VEGF). VEGF is an endothelial cell-specific mitogen, an angiogenic inducer, and a mediator of vascular permeability. VEGF has also been shown to be important for glomerular capillary repair. VEGF binds as a homodimer to one of two homologous membrane-spanning tyrosine kinase receptors—the fms-like tyrosine kinase receptor (Flt-1) (also known as vascular endothelial growth factor receptor 1 or VEGF-R1), and the kinase domain region receptor (Flk/KDR) (also known as vascular endothelial growth factor receptor 2 or VEGF-R2) which are differentially expressed in endothelial cells obtained from many different tissues. Flt-1, but not Flk/KDR, is highly expressed by trophoblast cells which contribute to placenta formation. Placenta growth factor (PlGF) is a VEGF family member that is also involved in placenta development. PlGF is expressed by cyto- and syncytiotrophoblasts and is capable of inducing proliferation, migration, and activation of endothelial cells. PlGF binds to as a homodimer to the Flt-1 receptor, but not the Flk/KDR receptor. Both PlGF and VEGF contribute to the mitogenic activity and angiogenesis that are critical for the developing placenta.
A soluble form of the Flt-1 receptor (sFlt-1) has been identified in a cultured medium of human umbilical vein endothelial cells and in vivo expression was subsequently demonstrated in placenta tissue. sFlt-1 is a splice variant of the Flt-1 receptor which lacks the transmembrane and cytoplasmic domains (He et al., Mol Endocrinol 13:537-545, 1999; Kendall and Thomas, Proc Natl Acad Sci USA 90:10705-10709, 1993).
Recent work by researchers at Beth Israel Deaconess Medical Center and Harvard Medical School has demonstrated increased placental production and maternal serum levels of sFlt-1 in patients with preeclampsia (Ahmad and Ahmed, Circ Res 95:884-891, 2004; Chaiworapongsa et al., Am J Obstet Gynecol 190:1541-1547, 2004; Koga et al., J Clin Endocrinol Metab 88:2348-2351, 2003; Maynard et al., J Clin Invest 111:649-658, 2003; Shibata et al., J Clin Endocrinol Metab 90:4895-4903, 2005). sFlt-1 is able to bind both VEGF and PlGF. Free in serum, it may diminish binding of these growth factors to their cognate receptors Flt-1 and Flk/KDR respectively (Kendall et al., Biochem Biophys Res Commun 226:324-328, 1996). In addition to VEGF and PlGF, the placenta is known to produce a number of other angiogenic factors, including the angiopoietins (Ang-1 and Ang-2) as well as their receptor Tie-2 (Dunk et al., Am J Pathol 156:2185-2199, 2000; Geva et al., J Clin Endocrinol Metab 87:4213-4224, 2002; Goldman-Wohl et al., Mol Hum Reprod 6:81-87, 2000). Increased levels of sFlt-1 and decreased levels of VEGF and PlGF are found in serum of women with preeclampsia.
Recent attention has also focused on another factor, endoglin (Eng), a co-receptor for transforming growth factor β1 and β3, and a protein expressed in large quantities by the placenta in preeclampsia. The extracellular domain of endoglin may be shed and is found in the serum where it is referred to as soluble endoglin (sEng). Like sFlt-1, sEng is increased in maternal serum 2 to 3 months prior to the onset of disease (Levine et al., N Engl J Med 355:992-1005, 2006).
The identification of an imbalance of circulating angiogenic factors that precedes the onset of preeclampsia or its clinical manifestation will be useful in designing screening and/or diagnostic tests to identify patients at risk for preeclampsia. Such a test would be invaluable to clinicians who may offer close follow-up and therapeutic interventions early in the course of disease. Several retrospective studies using serum obtained from patients having been afflicted with preeclampsia have shown that sFlt-1 concentrations in serum are high as much as 5 to 6 weeks before any clinical findings are noted (Chaiworapongsa et al., J Matern Fetal Neomatal Med 17:3-18, 2005; Hertig et al., Clin Chem 50:1702-1703, 2004; Levine et al., N Engl J Med 350:672-683, 2004; McKeeman et al., Am J Obstet Gynecol 191:1240-1246, 2004). In addition, free VEGF and PlGF are low (Hertig et al., Clin Chem 50:1702-1703, 2004; Levine et al., N Engl J Med 350:672-683, 2004). A recent systematic review of the literature to assess if elevated sFlt-1 or decreased PlGF in the serum could accurately predict the onset of preeclampsia concluded that third trimester increases in sFlt-1 and decrease in PlGF are associated with preeclampsia but there is currently insufficient data to recommend these as screening tests (Widmer et al., Obstet Gynecol 109:168-180, 2007).
A need continues to exist for more efficient and/or more effective methods of predicting a woman's risk for developing preeclampsia or determining if a woman has preeclampsia. Predicting and/or detecting preeclampsia in an early stage of gestation and/or in a stage of the disease where clinical evaluation may be uninformative would be particularly advantageous.